Photodimers, reversible

Photoreaction can remove thymidine-thymidine photodimers. Visible light activates the enzyme DNA photolyase. This enzyme absorbs visible light and transfers that energy to the photodimer, reversing the dimerization directly. 

Reversible Photodimerization Annealing of the Thin Film Powder X-ray Diffraction Thermal Analysis of the Thin Films Isomers of the Photodimers Reverse Reaction in Spin-Coated Film Crystal Structure and Photodimerisation... 

Formation of the photodimer is reversible, the dimer being split with high quantum yield with light of wavelength about 235 nm.(82> Similar dimerizations have been observed with several other pyrimidine derivatives, as... 

It is well known that pyrimidine bases convert to photodimers upon irradiation to UV light near the X max( > 270 nm). This photochemical reaction has a lethal effect in biological systems due to the photochemical transformation of pyrimidine bases of nucleic acids. However the photodimerization is a reversible reaction and the photodimers split to afford the original monomers very efficiently upon irradiation at a shorter wavelengths as shown in Scheme 1(1). 

The photodimer (Scheme 27) of acridizinium ethylhexanesulfonate has been used as a non-destructively read-reversible optical memory material to control the intensity and wavelength of emission from a laser (72MI21000). 

The appearance of at least two fluorescence lifetimes for cations 1 and 2 indicates that more than one excimer is present in the reversed micelle solution (Table 4). The percent contribution of the lifetime components change with . This finding is consistent with the notion that the size of the water pool affects the photochemistry of the stiibazoiium cation. At this point we do not have enough information to determine whether or not the different excimer lifetimes represent different states which lead to the production of different photodimers. 

A similar reversible formation of a photodimer from an alkanethiol-substituted coumarin monolayer is described by Fox and coworkers342. Irradiation at 350 and 254 nm effects dimerization and dissociation of the dimer, respectively. 

Examples of cycloaddition arising by a [ 4 + 4] pathway are, as usual, relatively rare. Pyrid-2-one has recently been shown to yield four [ 4 +, 4] stereoisomeric dimers on irradiation in aqueous solution. A micellar alignment effect has been observed in related A-oj-carboxyalkylpyrid-2-ones on irradiation in micellar and reversed-micellar systems, the ratio of cis trans dimers has been found to increase with decreasing length of the alkyl chain. In the corresponding 4-alkyl derivatives, only cw-dimers were obtained. The reversible formation of [ 4 -l- 4] photodimers of benzo[g]quinoline and A -methylbenzo[g]-quinolinium has been reported. 

In its deprotonated form, cinnamic acid already presents some prerequisites of surfactants and is easily incorporated in reversed micelles, as demonstrated bySawaki and co-workers " " and others.Then-results support the view that increased local concentration and alignment can be achieved at micellar interfaces. Substantially higher conversion of cinnamate to its photodimers is observed upon irradiation of reversed micelles formed by laurylammonium or hexadecyltrimethylammonium cinnamate in carbon tetrachloride. The photoproducts are mainly the P- and 5-truxinates, consistent with the requirements imposed by the assembly for the formation of contact ion pairs. Unfortunately, these are also the main products observed by Reiser upon irradiation of ethyl cinnamate (albeit in different proportions). Interestingly, however, the observed dimer distribution varies as a function of added water that is proposed to He near the ion pair interface. Similar results were also obtained for indene-2-carboxylate and using sodium dodecyl sulfate microemulsions in water. " The photodimerization of coumarin in micelles has been investigated by Ramamurthy and co-workers. ... 

The photodimer 34 is labile in fluid solution but thermally stable in the solid state at room temperature. This system has been proposed as a system for light-energy conversion, because the thermal reversion of the photodimer to the starting material 33 releases 116 kj mol. A similar reversible [2-t2]-cycloaddition has been reported for l,l -polymethylene-bisthymines (n = 2 to 6). The dimerization (A, = 300 nm) can be reversed by irradiation with 254-nm light. ... 

Reversible photodimerizations of l- -alkylthymine in single crystals depend on the crystal structures. However, the orientation of the thymine bases in the single crystal does not correspond to the isomer of the photodimer formed because thymines undergo disrotatory motion during the photodimerization process in the single crystals. The most important factor of the crystal structure for the photodimerization is the distance between the thymine base and the terminal methyl group of alkyl chain because this can influence the rotation of the thymine bases. 

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